As shown in FIGS. 1 and 2, a conventional laundry dryer includes a function control part 7 provided on the front surface of outcase 3 having a power switch 4 and a run/pause button 5 to input the function of a laundry dryer, a motor 6 mounted on the upper part of inside the outcase rotating a drum 1 according to the switching of the power switch 4 and the run/pause switch 5, a heater 2 mounted on a side inside of the outcase 3 for drying laundry, a drum temperature sensing part 8 provided outside of the drum 1 for sensing the inside temperature of the drum 1, a heat exchange fan 11 installed inside of the outcase 3, driven by the motor 6, cooling moist laden warm air inside of the outcase 3 by drawing in outside air through an air drawing in hole 15 and discharging warm air drawn from inside of the drum 1 to a discharge hole 14 in a drying passage 13, a discharge air temperature sensing part 9 sensing a discharge air temperature of the air discharged to the drying passage 13 of the outcase 3, an environment temperature sensing part 10 provided rear of the heat exchanger fan 11 sensing the temperature of the air drawn from the environment, a filter 12 filtering the air drawn-in from outside by the heat exchange fan 11 and a door 16 provided on the front of the outcase 3 for taking out laundry therethrough.
A conventional laundry dryer explained above is operated as follows;
When, after putting wet laundry into the drum 1 through the door 16 to dry wet laundry, a drying operation is carried out by turning on the power switch 4 and pressing the run/pause button 5 on the function control part 7. The motor 6 mounted on the upper part inside of the outcase 3 together with the drum 1 and the heat exchange fan 11 driven by the motor 6 start to rotate drawing in air through the air drawing in hole and the heater 2 starts to heat. The outside air drawn-in by the heat exchange fan 11 and passed through the heater 2 dries the laundry in the drum 1 slowly. That is, the warm air heated by the heater 2 vaporizes the moisture contained in the wet laundry in the drum 1 to become warm and humid air, which is delivered to the heat exchange fan 11 through the filter 12 positioned rear of the drum 1. The warm and humid air delivered to the heat exchange fan 11 exchanges heat with the cold air drawn-in from outside through the air drawing-in hole 15 by the heat exchange fan 11, the condensate by the heat exchange is drained through the drying passage 13 and the discharge hole 14, and the low temperature and low humid air after the dehumidification is blown through the drying passage 13 to the heater 2 to be heated again and delivered into the drum 1 to dry the laundry.
In this time, the drum temperature sensing part 8 positioned between the filter 12 and the heat exchange fan 11 senses the temperature T1 of the moist laden warm air which has dried the laundry, the discharge air temperature sensing part 9 senses the temperature T2 of the air after the dehumidification and cooling and the environment temperature sensing part 10 positioned in the rear of the heat exchange fan 11 senses the temperature 13 of the drawn in environment air.
That is, as shown in FIG. 3, the air temperature T1 inside of the drum 1 and the air temperature T2 after the dehumidification and cooling rise rapidly by the heater 2 within a preheat drying section t1 on the initial stage of drying operation while the environmental temperature T3 is the same all the time.
Thereafter, the air temperature T1 inside of the drum 1, the air temperature T2 after the dehumidification and cooling and environmental air temperature T3 do not rise any more maintaining the same temperatures all the time as the vaporization is at full-scale within the drying progression section t2, ie. equilibrium section after the preheat drying section t1.
Then, within the drying finish section t3 after the drying progression section t2, as the drying is almost finished, the air temperature T1 in the drum 1 rises again, the air temperature T2 after the dehumidification and cooling drops again, and the environmental temperature T3 is the same all the time. In this time, a temperature difference .DELTA.T1 between the air temperature T1 in the drum 1 and the air temperature T2 after the dehumidification and cooling is calculated, and when the calculated temperature difference .DELTA.T1 has been reached to preset temperature differences .DELTA.T2 and .DELTA.T3 preset according to the quantity of the laundry, judging that the dryness has been reached to 75%, 100% or more than 100%, the drying operation is finished by turning off the motor 6 and the heater 2.
However, such a conventional laundry dryer has a problem of having much electric power consumption because the heater used is of about 1300 watt capacity and the drying period of time required is normally about 2 to 3 hours. Further the dryer has a problem of damage of melting or heat deformation to some kind of laundry(especially wool and nylon) due to a drying temperature of about 60 degrees C. to 70 degrees C.